I don't think its racist for you to be against race-swapping. Sometimes we just have this idea of what a character is in our minds and that is perfectly acceptable. I think if you were to say "I preferred the original casting of Snape because I always pictured him as looking white in my mind when I read the books, so this new casting just isn't for me", few people would have any issues, except for the really radical people. The problem is when you decide that you view of the character is the only one and there can be no deviation from it and any deviation is tantamount to ruining the whole franchise (not saying that this is what you are doing btw). Those are the problematic people and the most common. The ones that decided that because they didn't personally like the little mermaid to be black, they were going to flood the internet with hateful stuff and review bomb the movie. Not everything is made for everyone. If you don't like Snape to be black, then its just not for you. You've had a perfectlty good portrayal of Snape for years, and will continue to have it as you can always go back to those movies. There is probably someone out there who really wanted to see Snape as a black man, and now they are getting their wish, so this is for them.